docs: Add --clean flag to coverage docs (#614)

runtime/manual/basics/testing/coverage.md

@@ -3,10 +3,8 @@ title: "Test Coverage"
 ---
 
 Deno will collect test coverage into a directory for your code if you specify
-the `--coverage` flag when starting `deno test`.
-
-This coverage information is acquired directly from the JavaScript engine (V8)
-which is very accurate.
+the `--coverage` flag when starting `deno test`. This coverage information is
+acquired directly from the JavaScript engine (V8), ensuring high accuracy.
 
 This can then be further processed from the internal format into well known
 formats by the `deno coverage` tool.
@@ -15,13 +13,26 @@ formats by the `deno coverage` tool.
 > immediately after running tests. Otherwise source code and collected coverage
 > data might get out of sync and unexpectedly show uncovered lines.
 
+The `--clean` flag has been introduced to the test runner
+[with Deno v1.44](https://deno.com/blog/v1.44#clean-coverage-directory-on-test-runs).
+This flag empties the coverage directory before running the test suite,
+preventing outdated coverage data from long-deleted files from lingering.
+However, be aware that this flag will cause conflicts when running multiple
+`deno test` commands in parallel or in series, and then viewing the aggregated
+coverage report. If you are running tests in parallel, you should not use this
+flag. If running in series, only pass this flag to the first `deno test`
+invocation.
+
 ```bash
 # Go into your project's working directory
 git clone https://github.com/oakserver/oak && cd oak
 
 # Collect your coverage profile with deno test --coverage=<output_directory>
 deno test --coverage=cov_profile
 
+# Use the --clean flag if you need to ensure old coverage data is cleared
+deno test --coverage=cov_profile --clean
+
 # From this you can get a pretty printed diff of uncovered lines
 deno coverage cov_profile